DC/DC converters are known from "Analoge Schaltungen", M. Seifert, VEB Verlag Technik Berlin, 1987, pp. 540 et seq. A DC/DC converter can be designed as a step-up converter, which produces an output voltage greater than the input voltage, or as a step-down converter, which produces an output voltage less than the input voltage. Such a DC/DC converter has a high efficiency. The ratio of output voltage to input voltage is determined by the duty cycle of the switching transistor.
An inductor serves as an energy store in the DC/DC converter. It generally consists of a coil with a ferrite core. In such a DC/DC converter, the inductor current is a function of the load current. When the inductor current exceeds a given value, the ferrite material quickly loses its permeability since the inductor becomes saturated, whereby a further increase in current is caused, which may result in the circuit or parts thereof being destroyed.
From data sheet MAX 731, MAXIM Integrated Products, Sunnyvale, Calif., USA, a DC/DC converter is known in which the inductor current is measured via a sensing resistor in series with the switching transistor. This makes it possible to measure any unwanted increase in inductor current and prevents a destruction of parts of the circuit.
This arrangement has the disadvantage that the sensing resistor must be large enough to produce a sufficient voltage drop for a precise current measurement. Hence, the efficiency of the DC/DC converter is reduced, since in the on state, the resistance of the switching transistor is increased by the sensing resistor. Viewed from this standpoint, a sensing resistor of zero resistance would be desirable.
Accordingly, it is an object of the present invention to provide a DC/DC converter in which the inductor current can be monitored without reducing the efficiency of the DC/DC converter.